Field Of The Invention And Related Art
The present invention relates to a charging device for charging (or discharging) a member to be charged or discharged, and more particularly to a contact type charging device (contact charging device or direct charging device) having a charging member contacted to the member to be charged and supplied with a voltage in use. The present invention also relates to a process cartridge and an image forming apparatus such as a copying machine or printer of an electrophotographic type or electrostatic recording type in which a charging member supplied with a voltage is contacted to an image bearing member to charge or discharge the imagebearing member in an image forming process.
The description of a conventional image forming apparatus for example, will be made for convenience of explanation.
Heretofore, in an image forming apparatus of an electrophotographic type or electrostatic recording type, a corona charger has been widely used to charge an image bearing member in the form of an electrophotographic photosensitive member or an electrostatic recording dielectric member or the like.
Recently, however, from the standpoint of the advantages of low ozone production or low electric power consumption or the like, a contact charging device having a charging member contacted to the member to be charged and supplied with a voltage, has been put into practice. Particularly, a roller type-charging device is preferably used because of the advantage of its stability.
In the contact type charging device of the roller charging type, an electroconductive elastic roller (charging member) is press-contacted to the member to be charged and is supplied with a voltage to charge it.
More particularly, charging is effected by electric discharge from the charging member to the member to be charged, and therefore, the charging action starts with a voltage at a threshold level.
For example, when the charging roller is press-contacted to an OPC photosensitive member having a thickness of 25 .mu.m (member to be charged), the surface potential of the photosensitive member starts to increase when a voltage not less than approx. 640 V is applied to the charging roller. Subsequently, the surface potential of the photosensitive member increases linearly with an inclination I relative to the applied voltage. Hereinafter, the threshold voltage is defined as a charge starting voltage Vth. Thus, in order to obtain a surface potential Vd of the photosensitive member required for an electrophotographic process, a DC voltage not less than Vd+Vth is required to be supplied on the charging roller. This is called a DC charging process since only a DC voltage is applied to the contact charging member.
However, it has been difficult to provide a predetermined potential level on the photosensitive member because the resistance of the contact charging member changes with variations in the ambient conditions, and because the film thickness of the photosensitive layer (member to be charged) is scraped which results in variations in the film thickness, which leads to variations in the threshold voltage Vth.
Japanese Laid-Open Patent Application No. 149669/1988 discloses, as a measure for providing more uniform charging, an AC charging system, in which an oscillating voltage includes a DC component corresponding to the desired Vd and an AC component having a peak-to-peak voltage not less than twice as high as the threshold voltage Vth. This is advantageous in that a potential uniforming effect by the AC is expected, and the potential of the member to be charged converges to the voltage Vd which is the center between the peaks of the AC voltage, and is not disturbed by any ambient condition change.
However, even is such a contact type charging device, the essential charging mechanism is based on the electric discharge from the charging member to the member to be charged, and therefore, the voltage required for charging has to be not lower than the surface potential of the member to be charged, which results in a small amount of ozone production.
When an AC charging system is used to provide uniform charging, noise (AC charging noise) produced by vibrations in the charging member and the member to be charged by the AC electric field, and deterioration of the surface to be charged by the discharging, are increased, which introduces new problems. Therefore, direct injection charging into the member to be charged, has been desired.
Contact injection charging, in which a voltage applied to a contact electroconductive member in the form of a charging roller, a charging brush, a charging magnetic brush or the like, is applied to inject electric charge to the trap level in the surface of the member to be charged, has been disclosed (see, e.g., "Contact charging property using electroconductive roller," Japan Hardcopy (1992, p. 287). In these methods, a photosensitive member (member to be charged) having an electrically insulative property in the dark is contact-charged by a low resistance charging member supplied with a voltage, and therefore, it is a premise that the resistance of the charging member is sufficiently low, that the material for imparting the electroconductivity to the charging member (conductive filler or the like) is sufficiently exposed at the surface.
Japanese Laid-Open Patent Application No. 57958/1976 discloses that a photosensitive member having a protection film in which conductive particles are dispersed, is electrically charged using conductive fine particles.
When direct injection charging is effected to the photosensitive member, it is required that the charging member and the surface of the photosensitive member are directly contacted ohmicly to permit transfer of electric charge therebetween, as contrasted to the conventional charging mechanisms using discharge. In other words, close contact between the charging member and the photosensitive member is required over the entire surfaces thereof, so that microscopic uncharged portions do not result.
In a conventional contact type charging system, the charging mechanism is based on electric description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.